This invention relates to an improved electrical control system which is particularly applicable to an automatic damper system in a fossil fuel fired heating plant.
In a typical such heating plant, a burner is controlled to operate whenever there is a call for heat and then shut down when the need for heat is satisfied. A thermostat would normally perform this control function.
The automatic system of the type herein described is designed to control the operation of a damper vane in the heating stack from the burner. When the burner is not in operation, the damper vane is maintained in a closed position to eliminate the so-called "chimney effect" whereby there is a steady flow of air through the burner and out the stack resulting in a steady loss of heat from the furnace.
When the thermostat calls for burner operation, the automatic damper system opens the heating stack by rotating or otherwise moving the vane to permit flow through the stack. At the end of the burning cycle, the vane is then moved back into the closed position, in some installations with a time delay to permit the complete elimination of the combustion products from the combustion chamber.
An automatic damper system of the type just described is shown in U.S. Pat. No. 4,039,123 in which the vane is maintained in an open position by a spring and held closed during the off part of the burning cycle by an electric motor. In this way, in the event of power or motor failure, the vane will be maintained in its open position, thereby insuring that no combustion products or fuel fumes will be trapped within the heating plant, the building or the residence.
An additional safety feature which is commonly incorporated into an automatic damper control system, including that of the system in the aforementioned patent, is an interlock switch which is directly actuated by the vane itself, so that when the vane is open, the switch is in one position, and when the vane is closed, the switch is in a second position. This switch, which may be referred to as an endswitch, has an override function, that is, regardless of the operation of the control system, if the switch indicates the vane is closed, even when the latter is supposed to be open, the electrical power to the burner will be interrupted.
The problem with the use of an endswitch performing such a function is that in time, as the system ages, or if the switch proves in use to be defective, the switch can fail to perform its override function.